The present invention relates generally to localized oxidation and more specifically to a method of reducing bird's heads in a localized oxidation process.
Localized oxidation which includes placing an oxide inhibiting mask over a semiconductor substrate and oxidizing the exposed portions of the substrate, has become a standard tool of the trade in fabrication of integrated circuits. One of the most popular masks is silicon nitride which is generally separated from the substrate by a thin oxide layer. One of the major uses of localized oxidation is to form a deep oxide region in the substrate which can be used for lateral isolation. One of the by-products of localized oxidation, is the lateral growth of oxide which produces a gradually increasing thickness of the oxide layer between the substrate and the nitride which is known as the bird's beak and a raised portion at the edge of the beak and the nitride mask which is known as a bird's head. Depending upon the specific structure and process steps, the bird's head may be in the order of 4,000-5,000 Angstroms in height.
Since it is generally preferable to have a substantially planar surface of the lateral oxide region and the substrate, the exposed portions of the substrate are etched to a given depth prior to oxidation. With significant bumps on the surface of the substrate produced by the bird's heads, subsequent oxide, mask and other layers must account for this uneveness and therefore masking accuracy and alignment are affected. This causes designers to use substantially more space than desired to assure the desired accuracy within the optical capability of the system.
Realizing the problems of bird's beaks and bird's heads, the prior art has offered many solutions.
U.S. Pat. No. 4,088,516 appears to solve the bird head problem by applying a vitrous solution to fill the undercutting of the silicon nitride layer so as to suppress lateral oxide growth. U.S. Pat. No. 3,900,350 reduces the bird's beak problem by providing a layer of polycrystalline silicon between the silicon substrate and the silicon nitride mask. U.S. Pat. No. 3,961,999 uses a chemical vapor deposition of silicon dioxide prior to thermal oxidation to minimize bird's beak and bird's head. U.S. Pat. No. 4,002,511 eliminates the bird's beak effect by forming the silicon nitride totally over the silicon dioxide layer so as to prevent lateral oxidation.
Generally, these processes include additional chemical steps or masking layers which increase the cost as well as affect the lateral area on the substrate which must be used to perform these additional steps. Thus, there exists the need for a process which does not use additional chemical steps to reduce the bird's head in a local oxidation process.
An object of the present invention is to provide a mechanical process for minimizing the bird's head effect in a localized oxidation process.
Another object of the present invention is to provide a process which minimizes bird's heads without introducing additional chemical steps.
A further object of the present invention is to provide a process which minimizes bird's heads without the addition of contaminant producing steps.
A still further object of the present invention is to provide a bird's head substantially planar with an oxide inhibiting mask.
These and other objects of the invention are attained by performing a standard localized oxidation process to form the local oxidation through an oxide inhibiting mask including the formation of bird's heads and grinding the bird's heads to a plane using the mask as a polishing stop. This standard oxidizing process includes forming an oxide inhibiting mask on the substrate generally of silicon nitride with a barrier surface between the silicon nitride and the substrate, forming openings in the mask to expose portions of the substrate and oxidizing exposed portions of the substrate. Preferably, the substrate is silicon with the barrier region being silicon dioxide. Prior to the oxidation, the exposed portions of the substrate are etched.